1. Field of the Invention
The present invention relates to high performance pneumatic radial tires. Particularly, the invention relates to improvement on the tread-reinforcing structure, and is aimed at prevention of the occurrence of buckling, improvement of the vehicle-cornering stability and reduction in generation of noises.
2. Related Art Statement
FIG. 2 shows a tread-widthwise sectional view of a part of a tread-reinforcing structure of a tire of this kind. In FIG. 2, a carcass 31 is formed from one ply of cords extending substantially at an angle of 90.degree. with respect to the tire circumferential direction, and a belt 32 is arranged on a radially outer side of a crown portion of this carcass 31 over an entire width of a tread. The belt 32 is formed by two belt layers 32a and 32b constituted from metal cords symmetrically arranged at relatively small angles with respect to a tire-circumferential direction such that the metal cords in one of the belt layers cross those in the other belt layer. On a radially outer side of the belt 32 is arranged an additional layer 33 constituted by a so-called cap and so-called layers, the cap being an additional reinforcing layer 33a composed of organic fiber cords extending substantially in the tire-circumferential direction and having a width, for example, slightly greater than that of the belt, and the layers being additional auxiliary layers 33b composed of similar cords extending in the tire-circumferential direction and covering axially side portions of the belt 32, respectively.
In FIG. 2, a treading surface 34 of the tread is formed with circumferential grooves 35 extending in the tire-circumferential direction.
In such a reinforcing structure, mainly the additionally reinforcing layer 33a prevents a central area of the tread from radially outwardly projecting due to centrifugal forces during high speed turning of the tire, and mainly the additionally auxiliary layer 33b prevents the axially side portion of the tread from radially outwardly projecting due to such centrifugal forces during high speed turning. Thereby, durability of the tire at high speeds can be improved.
However, in the case of the tire having such a conventional structure, particularly in which the belt layers 32a and 32b are made of metal cords, a so-called buckling phenomenon occurs that a widthwise central area of the belt 32 is deformed in a ground-contacting area of the tread in such a direction as to go away from the load upon receipt of a load there. Consequently, the ground-contacting pressure of the treading surface 34 of the tread becomes particularly low at a portion sandwiched between the widthwise central area and the circumferential groove provided on each of 7 widthwise outermost sides of the tread. Such a tendency becomes more conspicuous with increase in load.
For this reason, when the road-gripping forces of the tire becomes once lower, the ground-contacting pressure of the tire becomes non-uniform in the width direction of the tread during rotating of the vehicle as mentioned above. Consequently, the road-gripping forces are obliged to rapidly drop, so that tire controllability is largely deteriorated.
Furthermore, in such a conventional tire, since the additional reinforcing layer 33a reinforcing the central portion of the tread increases rigidity of the tread due to its large share of a tension load with the cords extending in the tire-circumferential direction, and causes resonance of the tire, noises of the tire generated unfavorably become greater.